1. Field
The following description relates to an optical communication system, and more particularly, to a control apparatus and method for controlling an optical receiver having delay paths.
2. Description of the Related Art
Formats used to generate a high-speed modulated optical signal include Non-Return-to-Zero (NRZ) and Return-to-Zero (RZ), in each of which data is coded on the intensity of an optical signal using a binary code (on or off), and Phase Shift Keying (PSK), Differential Phase Shift Keying (DPSK), and Differential Quadrature Phase Shift Keying (DQPSK) modulation schemes, in each of which a phase of an optical signal is modulated.
As a data transmission speed is increasing, more photo-electric frequency characteristics are needed. To this end, various methods have been introduced to implement a photo-electric or an electric-photo device that requires relatively small signal electrical bandwidth and has an improved data transmission speed by increasing the number of bits being transmitted per symbol. In this regard, DPSK scheme, in which a phase of light is modulated for transmission, in comparison to On-Off Keying schemes, such as NRZ and RZ, in which the intensity of an optical signal is modulated, has been more widely exploited to generate a high-speed optical signal and long-distance transmission, and related research and development have been continuously carried out.
The amplitude of a differential-phase-modulated optical signal is constant, and a phase difference between two neighboring bits has a value of either 0 or p. When a photo-diode directly detects the phase-modulated optical signal, the optical signal needs to be subject to demodulation in which the phase-modulated optical signal is converted into an intensity modulated optical signal. An optical receiver for demodulating a phase-modulated optical signal having information coded on the phase consists of a delay optical path structured in a delay Mach-Zehnder interferometer, a photo-diode, and an amplifier. However, such configuration leads to a structural unbalance in the optical receiver.